This invention relates to the field of acousto-optical interaction and, particularly, to a new method and apparatus for utilizing the phase change information and the stronger signal level of the zero order of diffracted light to visualize the longitudinal characteristics of acoustic fields.
The field of acousto-optical research has greatly burgeoned in recent years as the use of ultrasound information has found a great many new and important applications in medical and other areas. With this heightened interest has also come the realistic need to more fully understand the interaction of acoustic wave energy with bodily tissues and with other objects and materials in general. To aid in this work, the need has also heightened to be able to accurately and completely visualize sound fields both in their natural state and when interfaced with objects of varied dimension and material.
As examples of work in this area, Whitman, U.S. Pat. No. 3,633,407, discloses a sound field detection principle involving the displacement of a flexible pellicle surface P.sub.1 analogous to the RCA Sonovision system described below. The Whitman device is similar in technique to that used in the acoustic microscope as is the system shown in Ernvein, U.S. Pat. No. 3,829,827. The Ernvein patent discloses a typical holographic technique using a liquid surface for the acoustic image detection.
In like regard, Gabor, U.S. Pat. No. 3,745,814, and Korpel, U.S. Pat. No. 3,706,965, disclose acousto-optical surface detection systems somewhat similar to the RCA system and the Sonoscan microscope. Erikson, U.S. Pat. No. 3,990,296, and Brenden et al., U.S. Pat. No. 3,683,679, on the other hand, disclose somewhat analogous acoustic imaging systems also relying upon surface modulation to achieve the ultimate image. In this regard, Brenden et al. describes the interaction of two coherent beams on an object 79 at position 99 within a cell 15. The particles in the cell are caused to have local changes in density thereby producing diffraction, but the system lacks any phase-sensitive detector to display this information.
Mezrich et al., U.S. Pat. No. 3,997,717, is the RCA Sonovision system first developed at the Princeton Laboratories for use in acousto-optical investigation. The system was designed for visualizing transverse sections of acoustic wave fields and contains a scanning Michelson interferometer which measures the displacement of a thin, transparent metallized pellicle. The principle of the system is clear. An acoustic wave field is propogated through a medium and impinges at normal incidence on the back of the pellicle surface thereby establishing a standing wave field and theoretically causing the pellicle to respond as a particle of the medium to the acoustic field propogating through it. The Michelson interferometer provides a means for measuring the displacement of the pellicle surface in the standing wave field including a laser light source producing a beam first split into a reference and a target portion. The target portion impinges the flexible pellicle surface and the two portions are then later recombined and detection made of the change in path length of the target portion caused by the oscillating pellicle surface. The variations in path length are then used to detect the phase change information embodied in the interference pattern between the two beam portions. Mezrich et al. ('717) further theorize that the acousto-optical interaction between the ultrasonic field passing through the pellicle and the laser light portion measuring the pellicle displacement is insubstantial and does not significantly interfere with the displacement measurements.
Mezrich et al., U.S. Pat. No. 3,969,578, and Green, U.S. Pat. No. 3,716,826, are both forerunners of this RCA Sonovision system. Mezrich et al. ('578) also describes the use of the Michelson interferometer with a wiggling mirror as the means for varying the optical path length of the reference beam portion thereby providing a means of stabilizing the interferometer readings. Green discloses a similar but earlier system. As with Mezrich et al. ('717), however, both patents rely upon the physical measurement of the changes in path length of the laser light caused by the displacement of a flexible pellicle surface insonified from the back by a standing wave front as the basic principle of their sound field detection systems.
For the measurement and investigation of longitudinal wave information, systems utilizing Schlieren optics have been used for several years to study the performance of transducers and other ultrasonic devices. The principle underlying these systems is the use of first order diffracted light after acoustic interaction with a wave field in a transparent medium. A light source is used to transmit a coherent beam through a propogating acoustic wave field, or phase gradient. The beam is thereby diffracted into various orders (light components) and once through, the zero order is intercepted with a stop allowing the higher order acoustically diffracted component to be viewed. Although there is information in both phase and amplitude, the Schlieren technique then uses only the amplitude information, i.e., intensity, of the higher order diffracted light components to visualize the longitudinal character of the acoustic field. It totally disregards both the phase change information in the higher order components as well as the amplitude and phase change information in the zero order.
In this regard, Bhuta et al., U.S. Pat. No. 3,836,950, discloses a method utilizing acousto-optical interaction similar to the Schlieren technique. By positioning an apertured opaque mask 38 behind the lens 26a, the mask is able to intercept both the transmitted light-beam carrier and one first order diffracted side band component. Only the one remaining first order side band is thereby allowed to pass through the aperture in the mask for detection and imaging purposes. The system has no instrumentation for phase detection and relies solely on the amplitude of this single first order diffracted component to produce the acousto-optical image.
Alder, U.S. Pat. No. 3,431,504, describes a Bragg diffraction cell used to deflect a laser light beam as the interacting acoustic frequency is varied. The system then selects the desired diffracted order of the transmitted and deflected light pattern for subsequent translation and demodulation by the apparatus. The underlying purpose of the system is not at all concerned with investigation and visualization of sound fields as in the above-described prior art. Instead of attempting to describe the acoustic field, the Adler patent attempts to solely deflect the transmitted laser light and thereby modulate the angle of such light with a propogating acoustic beam. Moreover, this modulation of the angle does not occur with the zero order component.
As the field of ultrasound rapidly expands in the diagnostic, therapeutic, research and other areas, so too does the need for more accurate and sensitive methods and equipment for studying the acousto-optical interactions. In this regard the Schlieren imaging technique described above is hard-pressed to meet these ever-increasing demands. Relying solely upon the amplitude information of the diffracted first order light component to visualize the acoustic field, the system is lacking in the sensitivity and versatility required both for present and future applications. The need therefore exists for a new and improved system for visualizing the longitudinal and total character of acoustic wave fields being sufficiently sensitive and versatile to adapt to both present-day and future requirements.
In this regard, none of the above-cited references utilize zero order Raman-Nath diffraction to visualize the longitudinal or other characteristics of acoustic wave fields. In fact, the Adler reference specifically excludes the zero order component from the group of "orders" desirable for selection and examination. The same is true in Whitman and Bhuta et al.